It is known to have food products such as rations for back-packer, adventurers and military forces which comprise sealed packages of food. Conveniently, the food is hermetically sealed, within a vacuum; the food can be heated, on demand, since the food will already have been cooked. Such food products and apparatus are collectively referred to as Meals-Ready-to-Eat (“MRE”). In addition to the above uses, the products can be used as emergency rations for shelters and for heating of non-food products. Some of these meals are claimed to have a shelf life of 18-36 or more years from production. Typically food sachets are provided on a personal basis, since heat transfer issues may arise in larger packages. Food is heated to a sufficient temperature—around 70° C. or more—to make it a more pleasurable experience, compared with eating such food cold, which can incidentally, be done having already been cooked. Furthermore, there is a danger of burning when the outer wall of the exothermic material storage portion is not sufficiently thermally insulated. Accordingly such foods are typically prepared by heating with a flameless heater, frequently referred to as a flameless ration heater.
A flameless ration heater, or FRH, is a water-activated exothermic chemical heater included with Meals, Ready-to-Eat (MREs), used to heat the food. US military specifications for the heater require that it be capable of raising the temperature of a 250 gramme entree by 40° C. in twelve minutes, and that it displays no visible flame. A typical flameless exothermic chemical reaction works by an oxidation-reduction process, similar to the process of rusting metals—only far quicker. In the case of the flameless ration heater, one metal that can be used is magnesium. Magnesium is frequently chosen because it is readily oxidized and is mixed together with a small amount of iron in a pouch. To activate the reaction, a small amount of water is added, and the boiling point of water is quickly reached as the reaction proceeds. The iron plays a catalytic role in the heater mix Mg+2H2O→Mg(OH)2+H2. Salt water can be added to this composition.
The idea behind a flameless heater is to use the oxidation of a metal to generate heat. One type of flameless heater comprises magnesium dust which is mixed with salt and a little iron dust in a thin, flexible pad about the size of a playing card. To activate a flameless heater, water would be added. Within seconds of the addition of water, a typical flameless heater reaches the boiling point and is bubbling and steaming. To heat a meal in an MRE package, the emergency services personnel/explorer/soldier simply inserts the heater and the MRE pouch back in the box that the pouch came in. Ten minutes later, dinner can be served—typically from the same container in which the product was heated. Calcium oxide is another material commonly used in the manufacture of exothermic mixtures for MRE compositions, which is mixed together with sodium carbonate and aluminium.
FRH packages are quite expensive to produce and necessarily do not provide sustained heating effects. Additionally, the packages need to be transported with care and limitations are often provided when there is a high ambient humidity.